Process for electrodeposition of paint coatings on zinc-plated steel sheet



March 17, 1970 momu uc ETAL 3,501,278

7 PROCESS FOR ELECTRODEPOSITION OFvPAINT commas ON ZINC-PLATED STEEL SHEET Filed March 29, 1968 2 Sheets-Sheet 1 Paint Ep y Typ Concentration--40 'l- .p Chromium Temperature-----25'c Voltage 60 v 50- Phosphotized Steel Nickel 30- -Cupper Steel Paint Coating Weight(mgldm) 0' l I I Electra-Deposition Time(Sec.)

INVENTORS IQflMU (la/W04 IIAJIME WAG/N0 BY ku/v/A'Mv YOKOYAMA March 17, 1970 HIROMU UCHIDA ETA!- 3,501,278

PROCESS FOR ELECTRODEPOSITION OF, PAINT COATINGS ON ZINC-PLATED STEEL SHEET Filed March 29, 1968 2 Sheets-Sheet 2 Relation Between Zn Coating Weight and Corrosion Resistance 168- Chromium Coating Weight 5mg/dm' Red Rust Formation Time( Hour 11.4- Elect ro-Coating Conditions Epoxy Type Paint 90v 25min.

Baking Condition 96- 160 c 25min.

Zn Coating Weight (g/m') INVENTORS Maize.

United States Patent PROCESS FOR ELECTRODEPOSITION OF PAINT COATINGS ON ZINC-PLATED STEEL SHEET Hiromu Uchida, Hajime Nagino, and Kunihiko Yokoyama, Himeji-shi, Japan, assignors to Fuji Iron & Steel Co., Ltd., Tokyo, Japan Filed Mar. 29, 1968, Ser. No. 717,293 Claims priority, application Japan, Apr. 3, 1967, 42/ 21,103 Int. Cl. C23f 17/00 US. Cl. 29195 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for the electrodeposition of paint coatings on Zinc-plated steel and products obtained therefrom.

The zinc-plated steel products of the present invention have about 0.2 g./m. or more of zinc plated thereon. The coating Weight throughout the specification and claims is shown as that on one side of the material.

As is well known, the electrodeposition paint-coating process is a paint-coating process whereby coating material such as that of epoxy type, acrylic type, phenolic type, alkyd type, etc., dispersed or dissolved in an electro paint-coating solution, is electrodeposited on an article or product, e.g., zinc-plated steel. The article to be coated is placed in the solution or dispersion of the paint, using the article as anode with direct current passed thereto. This causes paint particles to adhere onto the article by electrophoresis.

Since zinc plated onto a steel surface effectively protects the steel from corrosion, zinc-plated steel products have been widely used for a long time. But steel products with only zinc plating are corroded in a relatively short period of time and adhesion of paint coatings thereonto is not entirely satisfactory. Therefore, as a surface treatment for the zinc-plated steel products such chemical treatments as phosphate treatment and/ or chromate treatment, etc., has been applied thereto.

In case an organic coating is applied by the above mentioned electrodeposition coating process to zinc-plated steel products with or without chemical treatment, the plated zinc and/or the chemically treated film are dissolved from the article when it is dipped into the paintcoating solution or dispersion and the article is electrically charged. This deteriorates the aqueous solution of the coating material. Also, this causes the coating material to be electrodeposited directly onto metallic zinc, which lowers the adhesion of the paint coating. Thus the electrodeposition of a paint coating over zincplated steel products for corrosion resistance has been considered disadvantageous.

In view of the above, it is an object of the present invention to provide a process for the electrodeposition of paint coatings on zinc-plated steel products wherein such coatings can be effectively applied and the adhesion of the paint coating onto the products is improved.

The above objects are attained by applying a chromium plating to zinc-plated steel products for a short period of time, prior to the electrodeposition of the paint coating.

Zinc plating and chromium plating in the present invention can be by any of the methods conventionally known. The steel products should be washed with water after zinc plating and the chromium plating should preferably be done immediately thereafter. It is desirous to send the chromium plated steel sheet to the paint electrodeposition tank after water rinsing without drying the plated material. By doing so, the chromium-plated material can be paint coated by electrodeposition while it still preserves its chemically active state. This provides good adhesion of the paint coating and results in good corrosion resistance and workability.

The product obtained by the present invention has such advantages that red rusting is satisfactorily prevented because even when the coated film of paint is damaged the steel surface is protected by the zinc existing as underlaying film.

The present invention shall be described referring to the attached drawings.

FIG. 1 is a diagram showing the relationship between the electrodeposition time and the thickness of electrodeposited paint coating film when the electrodeposition coating is given to various materials. FIG. 2 is a diagram showing the relationship between tht zinc coating weight and corrosion resistance of zinc-plated steel sheet with electrodeposition paint coating.

Table 1 shows the amount of zinc dissolved from the zinc plating into the aqueous electrodeposition paint solution from steel which had been zinc plated and then chromium plated, and from steel which had been zinc plated and then chemically treated.

Conditions for chromate treatment:

Sodium dichromate g./l. Sulphuric acid-03 g./l. Nitric acidl.0 g./l. Temperature 5050 C. Time5 seconds.

Conditions for phosphate treatment:

Bonderite #37 (commercial name, available from Hooker Chemical Corp.) is used.

Free acid-1.1

Total acidity-15.4

Acid ratiol4 Concentration35 g./l.

Temperature-50 C.

Time2 minutes.

Conditions for electrodeposition coating:

Epoxy type electrodeposition coating material is used. Voltage9 v. Time3 minutes. Coating temperature-35 C.

Conditions for zinc plating: Zinc sulphate250 g./l. Sulphuric acid--l.8 g./l. Aluminium sulphate g./l. Temperature-40 C.

Current density30 A./dm.

TABLE 2 Paint Adhesion Tests Erichsen Impactlest (weight-height) Scratch Cup Value Material Test (mm.) cm. cm. cm. em.

Electro galvanized steel sheet (zinc coating weight, 18 g./m. X 3.1 X X X X Electro galvanized steel sheet with chromate treatment applied thereon (zinc coating weight, same as above) A 7. 3 A X X X Electro galvanized steel sheet with phosphate treatment applied thereon (zinc coating weight, same as above) 7. 8 0 A A X Electro galvanized steel sheet with chromium plating applied thereon:

Zine coating Weight, 18 g./m. chromium coating weight, 1.0 mg. ldm. 8.0 O O O 0 Zinc coating weight, 18 g./m. chromium coating weight, 1.5 mg/dm. 8.0 0 O 0 O Zinc coating weight, 18 g./m. chromium coating weight, 2.0 mg./dm. 8.0 0 0 O O Zinc coating weight, 18 g./m. chromium coating weight, 4.0 mg dm. O 8.0 O O O O Zinc coating weight, 18 gJmfi; chromium coating weight, 6.0 mg. ldrn. O 8. O O O O O Galvanized plated steel sheet (zinc coating weight, 92 g./1n. with chromium plating applied thereon (chromium coating weight, 4.0 mg./dm. 7. 9 O O O 0 Paint Adhesion Tests After Immersion in Warm Water 1 Material Scratch Test Erichsen est 2 Impact Test (weight-height) Electro zinc plated steel sheet (zinc coating X weight, 18 g./rn. Electro galvanized steel sheet with chromate treatment applied thereon (zinc coating weight, same as Electro galvanized steel sheet with phosphate treatment applied thereon (zinc coating weight, same as above) 0 8 weight, 92 g./m. with chromium plating applied thereon (chromium coating weight, 4.0 mgJdmfl) OOOO OOOO

1 The tests were conducted at 37 C. for 1 week.

The test pieces were drawn 8 mm. deep with an Erichsen tester and peeling ofi of the organic coating was observed.

In the table the marks indicate as follows:

0 =No defects in the coating. A=Coating partly damaged.

X Coating damaged.

Thickness of steel sheet 0.8 mm.

The conditions for zinc plating, chromium plating eln'omate treatment, phosphate treatment, and clectrodeposition coating are same as those in Table 1.

The baking after electro coating was done for 25 minutes in an electric furnace at 160" C.

In impact test, DuPont type tester with a weight of 1 kg. and a punch die with $4 diameter was used.

Conditions for chromium plating:

Chromic acid-100 g./l. Sulphuric acid-1.1 -g./l. Temperaturc50 C. Current density35 A./dm.

On the other hand, FIG. 1 shows the relationship between the electrodeposition time and the thickness of elec- The above table shows that the amount of chromium plated, about 1.5 mg./dm. or more is desirous. The maximum amount of chromium plated should be about 7 mg/drn. from a standpoint of workability. More than 7 m n/dm. of chromium coating cracks when it is bent or drawn. As to the amount of zinc plated, it may be such amount as ordinarily used in conventional zinc-plated steel sheet, that is about 9 g./m. to g./m on electrogalvanized steel sheet and about 30 g./m. to 230 g./m. on galvanized steel sheet (both one side), but it can be such amount as far less than those, that is, about 0.2 g./rn. The zinc plated in an amount smaller than that will result in inferior corrosion resistance, as shown in FIG. 2.

FIG. 2 shows red rust formation time on the cross-cut portions of steel sheets having ditferent amount of zinc plated thereon, with a certain amount of chromium plating and electrodeposition paint coating thereon, which were cross-cut on the surface and was salt Water sprayed.

FIG. 2 indicates that when the amount of Zinc plated is more than about 0.2 g./rn. the corrosion resistance after electrodeposition coating is good.

It was confirmed by electrode potential measurement in 0.1 NH SO that when the amount of zinc plated is less than 0.2 g./m. Zinc potential was not shown.

That is, the steel sheet, which has more than 0.2 g./m. of zinc plated thereonto and 1.5 to 7.0 mg/dm. of chromium plated thereover and then is subjected to electro-deposition coating, has excellent paint coating adhesion, corrosion resistance and workability.

What is claimed is:

1. In a process for the electrodeposition of paint on zinc-plated steel, the improvement which comprises plating chromium on the Zinc-plated surface of said steel and electrodepositing the paint on the chromium plating.

2. A process of claim 1 wherein the steel is rinsed with Water after chromium plating and subjected to the electrodeposition of the paint while still wet from said rinsing.

3. A process of claim 1 wherein the amount of zinc plating is greater than about 0.2 g./rn. and the amount of chromium plated is about 1.5 mgjdm. to 7.0 mg/ dm.

4. A process of claim 2 wherein the amount of zinc plating is greater than about 0.2 g./m. and the amount of chromium plated is about 1.5 mg./dm. to 7.0 mg./ dm.

5. A process of claim 1 wherein the amount of zinc plating is about 0.2 g./m. and the amount of chromium plated is from about 1.5 mg./drn. to 7 mg./dm.

6.'A process of claim 2 wherein the amount of zinc plating is 0.2 g./m. to 9 g./m. and the amount of chromium plated is from about 1.5 mg./dm. to 7 mg./ dm.

7. A process of claim 2 wherein the paint coating is selected from epoxy, acrylic, phenolic or alkyd type coatings.'

8. A steel article having an electrodeposited paint coating prepared by the process of claim 1.

References Cited UNITED STATES PATENTS 8/1957 Hurd et a1. 174110 12/1946 Tanner 204 Mason 20438.41

US. 01. XR. 204--38 

